Do you have any proof to claim that you didn't try arbing with the bookmaker?
Yeah, I understand no one may take screenshot of their bets so they can use it as proof in case if there is something wrong but without proof you can't blame them and have to accept what the casino says.
It's the casino who are accusing op of arbring. So, it's them who have to prove this. As always, the burden of proof lies on the accuser!!
We all know that casinos will never disclose any information from their investigation, for obvious reasons!
The good thing here is that op got his money back. If he is not satisfied with this casino's services, then he should simply look for a better alternative.